Multi-channel communication system



Mardi 17, 1959 w. P. BooTHRoYD ET AL 2,878,370

MULTI-CHANNEL. COMMUNICATION SYSTEM March 17, 1959 w. P. BooTHRoYD ETAI.2,878,370

MULTI-CHANNEL COMMUNICATION SYSTEM Original FiledJan. 14, 1949 2Sheets-*Sheet 2 Frm/77 one Hua/o Fare/e F/Pom one mf an may nera/0M[ir/4.1]

, pany me pms:

|f @M I i i l Hy! Hua/o vnr/wa wmf:

on conf/m @ma $7 I 0F Pfnrooe +3K I i avi u max sup/maison I 0 I monarom-166 of Panraoe FVQAZ.

' aaa/)rs Y United Wilson P. Boothroyd, Huntingdon Valley, and Edgar M.Creamer, lr., Philadelphia, Pn., assignors to Philco Corporation,Philadelphia, Pa., a corporation ofPenu- -sylvania Original applicationJanuary 14, 1949, Serial No. 70,953,

now Patent No. 2,680,153, dated .lune 1, 1954. Divided and thisapplication May 28, 1954, Serial No. 433,042

3 Claims. (Cl. Z50-27) The present invention relates to a modulator.While the modulator of the present invention is particularly adapted foruse in a multi-,channel communication system of thepulse-amplitude-modulationV type, its use is not limited thereto.

This application is a division of an application tiled by the presentapplicants on January 14, 1949, Serial Number 70,953, which issued onJune l, 1954 as U. S. Patent No. 2,680,153.

A principal object of the present invention is to provide an improvedmodulator.

A more specific object is to provide a modulator suitable for use in apulse-amplitude-modulation communication system of the type describedand claimed in the parent application referred to above.

In the drawing of the present application;

Figure 1 shows a block diagram of a preferred form of multiplexcommunication transmitter system;

i Figure 2 is a circuit diagram of a preferred form of modulatorincluded in the transmitter system of Figure l; and

Figure 3 is a set of waveforms which will be helpful in explaining theoperation of the modulator of Figure 2.

Referring now to Figure l, there is shown a schematic block diagram of apreferred form of pulseamplitude modulated multiplex transmittingsystem. This system includes a 120 kilocycle oscilator Separating intimed relation with a pulse generator 10. The latter produces a seriesof uniform and uniformly spaced triangular pulses having a constantrepetition rate. While the pulse generator may be of any suitable typeknown in the art, one particularly appropriate design is described andclaimed in U. S. Patent No. 2,616,047, granted October 28, 1952 to W. P.Boothroyd.

The repetition rate of the pulses produced by the generator 10 is 8kilocycles. ln other words, the peak of each pulse is spaced in timefrom the peak of the immediately preceding pulse by an interval of 125microseconds. Furthermore, for reasons which will later become appar#ent, each triangular pulse has an effective width at its oase which isno greater than 8 microseconds.

These pulses from the generator 10, which may have a waveform such asshown in the drawing by the reference numeral l2, are applied to theinput terminal of a delay network 14. This network 14 may be of any formknown in the art, such, forexample, as a plurality of series-connectedinductors and shunt-connected capacitors arranged to form individualsections or units. Network 14 is provided with 30 equally-spaced outputtaps chosen so that the time delay for each section of the network isapproximately 4.16 microseconds. The total delay in-` terval for theentire network is thus 4.16-30, or 125 microseconds, and issubstantially equal to the period of the pulses 12. Such delay networksare known in the art, but one type of delay network which isparticularly suited for this purpose is shown in the aforesaid BoothroydPatent 2,616,047. i

In order that the delay network 14 may remove the tales Pater 2,878,370Patented Mar. 17, 1959` ice highffrequency components present in the`pulse output of the generatorv 10,` a low-pass lter is incorporated 14is of course determined by the particular values of` the inductors andcapacitors making up` the assembly. It has been found in practice that acharacteristic imped ance of 2500 ohms will produce satisfactoryresults, and a suitable` terminating impedance of this value is used.

One important characteristic of the delay network,` 14 is that it doesnot introduce any appreciable change; in the waveform of the pulses 16as they travel therealong. After the output pulses 12 from the generator10 have passed through the first few sections of the delay network 14(which constitute the low-pass filter), and have arrived at the trstoutput tap of the network with the shape shown at `16, no signilicantchange occurs; in the waveform of the pulses until after they passlthelast out; put terminal #30.

summarizing the above, a pulse 12 appliedto the input terminal of thenetwork 14 appears at the output taps #1-#30 with the waveform 16Vsuccessively at times spaced approximately-4.16 microsecondsapart. Thewave retains substantially this same shape at each output terminal ofthe network.

The transmitting system of Fig. 1 is designed to multi plex thirty audiochannels una time-sharing basis. This is accomplished by sampling theintelligence signal in each channel at a rate equal to at least twicethe high-` est frequency contained therein. detects the instantaneous`amplitude of the intelligence sig nal at theinstant when samplingoccurs, and, since the channels aresampled in sequence, the channelinforma-` tion'is available for intermixing Vinto a compositemultiplexed signal.

As the pulse 16 passes the various output taps of the network 14in.SCqUCJlC it becomes a timing wave for the4 purpose of sampling therespective intelligence channels..

The embodiment of the invention illustrated includes thirty suchchannels, although this number was arbitrarily` chosen and thus ismerely exemplary. One of these chaunels transmits an indexing tone `forIsynchronizing pur.-` poses and is also used as an order line. Theremaining twenty-nine channels are availablefor audio communication.`

Each of the thirty output` taps or terminals of the delay i network 14is connected to` one of thirty modulators 18. Twenty-nine of thesemodulators also receive sig,- nals from twenty-nine audio inputchannels, each of which includes a microphone 20 or other source ofaudio `frequency signals. In order that all frequencies outside the 300to 3,300 cycle range may be eliminated from the output of themicrophones 20, a filter 22 is provided i in each audio channel. Theoutput of each filter 7.2,

therefore, is an audio signal having no frequency higher thanapproximately 3,300 cycles per second.

Each of these audio signals is applied to its respective This samplingprocess` kilocycle A'energy appearing at 'the output of any particularvone of the modulators 18 will depend upon the instantaneous value of theaudio signal applied to that particular modulator at the instant when asampling pulse is also applied thereto. Thus the signal in each audiochannel may be transmitted without any appreciable loss of theinformation contained therein.

The single remaining modulator #27 receives both an indexing tone at afrequency of 3,900 cycles from a generator 24 and also the outputof anorder line ilter 26. Inasmuch as the order line information in theembodiment described does not require as high a frequency range as thatof the remaining audio inputs, the order line lter 26 (which isconnected toa microphone 28), has a frequency passband of from 300 to2,500 cycles. Since the highest frequency applied to the indexing toneand order line modulator is 3,900 cycles per second, the intelligence inchannel #27 willv still be sampled at least twice per cycle by the 8kilocycle timing wave 16.

`Although any one of the modulators 18 might have beenselected toreceive the combined output ofthe in-4 dexing tone generator 24 and theorder line filter 26,`

in the present embodiment channel #27 was selected for this purpose.Thus, each one of the thirty modulators 18 is connected to receive atriggering pulse in timed sequence from one of the thirty taps on thedelay network 14. i

' The respective outputs of the modulators 18, representing thirtychannels Aof amplitude-modulated pulses, are then combined into a singlemultiplexed signal by the combining circuit 30. The signal in the outputof the circuit 30 may have a Waveform such as represented by thereference numeral 32. This` wave 32 is a composite multiplexed signalcomposed of thirty phase-delayed pulses derived from each one of the 8kilocycle Atimingv pulses 16, or 240,000 amplitude-modulated pulses persecond. Each thirtieth pulse in this wave represents the intelligence ofone particular channel.

One of the principal features of the system shown and described in theparent application, Serial No. 70,953, of which this application is adivision, resides in the ability of the apparatus there disclosed tooperate with a bandwidth which is approximately equal to the normalspectrum of the multiplexed signal. It has been found that thetransmitted intelligence will be reproduced with negligible distortionif the 8 kilocycle sampling wave is transmitted with a substantialnumber of its harmonics, each of these harmonics having sidebandsextending a distance on each side thereof equal approximately to thehighest channel frequency. It has furthermore been found that anunmodulated reference wave representing the 240 kilocycle samplingfrequency is desirable at the receiver in order to aid in thedemodulation of the multiplexed signal.

According to this feature of the system shown and described in theparent applicant, a transmission bandwidth of only approximately 150kilocycles is required. Although this permits transmission of anappreciable number of the harmonics in the 8 kilocycle sampling wave, itdoes not permit transmission of the unmodulated 240 kilocycle wave.However, if a subharmonic of this 240 kilocycle frequency is derived atthe transmitter (such as 120 kilocycles, for example), it may be addedto the transmitted signal and then restored to its original form at thereceiver by utilizing a suitable frequencydoubling circuit.

In order that the multiplexed signal may be transmitted within theabove-mentioned 150 kilocycle passband, the composite signal 32 isapplied to a band-limiting network 34 which in effect consists of alow-pass lter having a response which drops only gradually up to 150kilocycles but then falls off sharply until it is down substantially 40db at 260 kilocycles. This band-limiting apparatus permitstransmissionof 'the`8 kilocycle sampling wave with `anurnber of harmonics (up to atleast the fifteenth), and, furthermore, passes the two sidebands of eachof these harmonics with substantially equal amplitude. However, it isrecognized that the cut-off of the band-limiting network 34, such asshown by the response curve 36 in Fig. 1, will introduce considerablecrosstalk into the signal 32 unless it is cornpensated for. The meansfor producing such a compensation are an essential portion of theinvention, and will be fully described in connection with a descriptionof the receiving apparatus, as set forth below.

A portion of the output of the oscillator 8 is applied to a 120kilocycle filter 38, which may also, if necessary, include suitableclipping and amplifying means for producing an output wave 39 ofconstant amplitude. Any necessary phasing of the wave 39 may be broughtabout by a phasing unit 40. The output of the unit 40 is combined inproperly timed relation with the output of the band-limiting network 34,and the resulting wave is emplo'yed to modulate either a transmitter 42or anyother type of translating device.

13 in Fig. 1. It will be appreciated that it is the purpose of each`such modulator 13 to act as a gating circuit l' which effectivelyconnects the output of its respective audio lter 22 to the modulatoroutput circuit (combining circuit 30) .in accordance with theapplication to the modulator of one of the timing pulses 16 from thedelay network 14., The outputs of the respective modulators are thenconsolidated as shown in Fig. l in the combining circuit 3th. It will befurther appreciated that the modulator 18, or gating circuit, must beclosed to the output of its associated audio filter 22 at all timesexcept when it is opened by the application thereto of one of the timingpulses 16 from the delay network.

Accordingly, each one of the modulators 18 in Fig. 1 may include apentode 76 (as shown inFig. 2) to the control grid 78 of which theoutput of an audio lter 22 is applied. The screen grid 80 of tube 76 isconnected to a source of positive potential through a resistor 82. Thecathode 84 of the tube is connected to ground through two seriesresistors 86a and 86b, and is also connected directly to the anode of adiode 88. The cathode of diode 88 is connected to a source of negativepotential through the resistor 89.

The pulses 16 from the delay network 14 are applied with positivepolarity to the suppressor grid 90 of the pentode 76 by means ofconductor 91 and condenser 92. The bias on suppressor grid 90 ismaintained at a selected negative potential in excess of cut off byhaving its leak resistor 92a connected to the arm of the potentiometer93 which is connected between a negative potential source and ground.The output of the pentode 76 is taken directly from its plate 947 and isapplied as shown to the combining circuit 30 (Fig. 1).

The audio voltage from the filter 22 in Fig. l is accordingly applied toone grid of the pentode 76, while the timing pulses 16 from the delaynetwork 14 are applied to another grid of the same tube. In this manner,the pentode is gated upon by the timing pulses so as to pass into itsanode circuit a voltage representative of the instantaneous amplitude ofthe audio signal during the presence l of the gating pulse. However, ithas been found that the amplitude of the output signals from anode 94will not be linearly related to the applied audio signal unless thegating voltage on its suppressor grid 90 can be made to vary as afunction of variations in the audio voltage appearing on its controlgrid 78 and hence on its cathode 84.

v V The means for bringing about such a correlation in theapplied tubepotentials includes the diode 88. Since the voltage onthenanode Aof thisdiode is substantially the However, it should be understood that thewave representing the combined out' puts of the band-limiting network 34and the phasing same as the voltage on its cathode under normaloperating conditions, because the diode is normally conducting, it willbe seen that the alternating component of its anode voltage (which isthe voltage on the cathode 84 of the pentode 76) will traverse condenser99 and appear on the suppressor grid 90 substantially without change.Hence, the gating voltage of tube 76 will in effect be superimposed uponthis varying cathode voltage, and thus will vary as a function ofvariations in the amplitude of the audio signal.

The division of current between the screen grid 80 and the anode 94 ofpentode 76, at a constant suppressor grid voltage relative to thecathode, is a function of the current in the triode section of the tube.Thus some curvature is introduced into its cathode current-plate currentcharacteristic which must be corrected if amplitude distortion of theaudio signal is to be avoided. This correction is brought about byintentionally predistorting to a selected degree the gridvoltage-cathode current transfer characteristic of the tube. Since thisintentional distortion is in an opposite sense to the cathodecurrent-plate current curvature, the two types of non-linearity may becaused largely to cancel one another, resulting in a com-` pensatedoverall characteristic which has substantially less undesired curvaturethan the uncompensated characteristic alone.

The circuit of Fig. 2 is designed as to include cathode degeneration andthereby to provide an amplitude-modulated output in which substantiallyno width modulation of the pulses is present. As shown by the waveform95 in Fig. 3, the anode voltage of tube 76 remains at a level ofapproximately 180 volts positive when no plate current is flowing. Thevoltage on the screen grid 80 is approximately +75 volts. The two seriesresistors 86a and 86b act to hold the cathode 84 at a potential ofapproximately 3 volts above ground as a result of the ow of screencurrent. Since the suppressor grid 90 is normally maintained at apotential of -55 volts, no plate current normally iiows in the tube.However, when the suppressor' grid 90 reaches a voltage of -10 voltswith respect to ground as shown in curve 96, current ows to lower thetube plate voltage as shown in curve 95. The cathode current isunchanged by this transfer of screen current to the plate.

A typical portion of the signal from the audio filter 22 is shown incurve 97, and is applied to the control grid 78. This control grid 78,the screen grid 80, and the cathode 84, act together as a triode toproduce an audio voltage at the cathode which varies between +1 and +5volts under peak input conditions. This voltage is conducted through thediode 88 to the suppressor grid 90 to swing the voltage on this gridbetween -53 and -57 volts, approximately.

The timing pulse 16 is also shown in Figure 3, and is present at thejunction 98 for a period of 8.33 microseconds in each cycle ofoperation. The positive voltage at this junction 98 drives thesuppressor grid 90 from -55 volts to +3 volts (as shown in curve 96) inthe absence of an audio voltage, or to between +1 and +5 volts if anaudio voltage is present. Because of the presence of this audio voltageon the cathode 84, the control grid 78, and the suppressor grid 90 ofthe pentode, the time at which the pulse 16 causes conductance tocommence in the anode circuit of the tube is definite and independent ofthe audio voltage. The same is true of the time at which conductanceceases. The amount of plate current which ows at the peak period of thepulse 16 depends upon the magnitude of the audio voltage, and is such asto cause a variation in the plate voltage of the tube from a nominalvalue of 178 volts to approximately 177 or 179 volts.

When the timing pulse 16 begins, it cuts off the diode 88. This preventsthe internal impedance of the pentode, and the impedance of its cathoderesistor, from placing a load on the delay network 14.

While the resistor 89, which is returned to volts, permits the dischargeof condenser 99 when the cathode 84 of tube 76 swings in the negativedirection after a positive excursion, nevertheless the time constant ofthis combination is so chosen that condenser 92 retains substantiallyall of its charge during the presence of the timing pulse 16. It is thischarge, as varied at an audio rate through the diode 88, that forms thebase, or pedestal, for the timing pulse 16.

Having described our invention, we claim:

1. In combination: an electron discharge tube having at least anode,cathode, and first and second control grids; a source ofamplitude-modulated signals; an impedance having substantial magnitudeat the frequency of said applied signals connected between said cathodeand a point of lixed reference potential; means for applying saidsignals between said first control electrode and said point of fixedreference potential to vary the potential of said cathode in accordancewith the instantaneous amplitude of said applied signals; a source ofpositive timing pulses; means for applying said timing pulses betweensaid second control grid and said point of fixed reference potential torender said tube conductive; and means including a diode and a capacitorconnected in series between said cathode and second control grid to varythe potential of said second control grid in accordance with variationsin said cathode potential, the anode of said diode being connected tosaid cathode of said electron discharge tube, whereby said tube isrendered conductive at intervals controlled by said timing pulsessubstantially independently of the amplitude of the said applied signalsduring said intervals.

2. A gating circuit for a time varying signal, said circuit comprising:a vacuum tube having cathode, anode and plural control grid electrodes;means for applying said signal to one of said control grid electrodes;means for producing, at said cathode, variations in potentialcorresponding to the variations in amplitude of said applied signal;means for applying to another one of said control electrodes a biaspotential of such magnitude and polarity as to maintain said tube cutoi; means for intermittently applying to said other control gridelectrode a potential of such polarity and amplitude as to overcome saidcut-oli:` potential and render said tube conductive; and means forsuperposing, on said potential applied to said other control gridelectrode, variations substantially corresponding to said cathodepotential variations, said last-named means comprising a unilaterallyconductive device and a charge storage means connected in series betweensaid cathode and said other control grid electrode, said unilaterallyconductive device being connected with such polarity as to be renderednon-conducting by the application of said intermittently appliedpotential to said other electrode and said charge storage means havingsuch charging time-constant as to become charged to substantially saidcathode potential during conduction of said unilaterally conductivedevice and having a suciently long discharge time-constant to remainsubstantially at said cathode potential during any one application ofsaid intermittently applied potential to said other control gridelectrode.

3. The apparatus of claim 2 further characterized by the provision ofmeans for applying to said unilaterally conductive device a biaspotential of such amplitude and polarity as to maintain said device inconduction except during the application of said intermittently appliedpotential.

References Cited in the le of this patent UNITED STATES PATENTS2,405,238 Seeley Aug. 6, 1946 2,549,780 Earp A pr. 24, 1951 2,591,088Millman et al. Apr. l, 1952 FOREIGN PATENTS 643,471 Great Britain Sept.20, 1950

